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Population Ecology Population Ecology •  Characteristics of individuals and

populations •  Life histories •  Population growth •  Population-limiting factors •  Population interactions •  Human Population growth

The Origins of Population Ecology

•  Thomas Malthus –  1798 published his

Essay on the Principle of Population. Malthus introduced the concept that at some point in time an expanding population must exceed supply of prerequisite natural resources, i.e., population increases exponentially resulting in increasing competition for means of subsistence, food, shelter, etc. This concept has been termed the "Struggle for Existence".

–  Influenced many thinkers…among them?

Chuck, post-Beagle

Characteristics of Populations

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Siphonophores Phylum Cnidaria Class Hydrozoa

Once we are clear on what is an individual, we can talk about populations

•  Some Characteristics of populations: –  Distribution: the area in which it occurs –  Abundance: # or size of population

•  Density: # / unit space –  Various ways to estimate, most are difficult

–  Rates of birth, growth, death, etc. –  Spacing/ Dispersion

•  Organisms, and therefore populations, respond to a variety of environmental factors.

•  Only when all are within tolerance can it live in an area, its Habitat

Spacing options ~Lets be random~

What about over larger areas?

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Sea Otter Distributions What about vertically… or in 3-D?

What does ‘random’ or ‘normal’ mean? Why do thing live were they do?

Moving from Individuals to Populations Demography

•  Factors affecting growth and decline of populations – Birth –  Immigration – Death – Emigration

•  Survivorship curves and Life Tables

Survivorship Curves

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Reproduction strategies

•  Semelparity (big-bang) – One time reproductive events – Examples?

•  Iteroparity (multiple) – Repeated reproduction – Examples?

What factors drive the evolution of these strategies? How many eggs do you put into one basket?

Reproductive trade-offs

Kestrels

Rates of Growth •  Some examples…

9,223,372,036,854,780,000

Exponential Growth

•  Exponential growth (geometric) –  Intrinsic rate of increase under ideal conditions

(not limited by anything) = rmax

dN/dt=rN

Logistic Growth

•  Real world growth is usually not exponential…for long.

•  Limited resources put many pressures on growing populations

•  Carrying capacity –  “K”

•  Maximum population size that a particular environment can support at a particular time, with no harm to the habitat.

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Carrying capacity and growth rates dN/dt=r [(K-N)/K]

Examples of real growth

Logistic growth breakdown

•  Assumptions of logistic model – Every individual has a negative effect – Populations adjust instantaneously – Limiting factor is always population size

•  Useful starting point in understanding very complex systems

K and r selected Species •  At high population densities (close to ‘K’)

– Selection favors adaptations that help organisms survive and reproduce with few resources

– Efficiency and Competition •  At low population densities

– Selection favors rapid reproduction and shorter lifespans

– Dispersal and Growth •  K-selection- density dependent •  r-selection- density independent

dN/dt=r [(K-N)/K]

Population limiting factors

•  Negative feedback – Difficult to study (often need long-term data) – Resource limitation (food, shelter, etc) – Predation pressure, toxic wastes, disease

Density-dependent changes

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Species interactions •  Classic example of lynx and hare •  Other Non-consumptive effects…

–  Reduced physiological condition, behaviors, etc •  50% + drop in fecundity for years after crash…

Siphonophore Abundance Human Populations •  http://www.census.gov/main/www/popclock.html

Why talk about humans in ecology section?

Human population  Earth's population reached 6 billion in September,

1999  Increase to 7 billion by end of this decade  Exponential growth of human population since

early 1800s  40,000 years for 1st billion, 100 years for 2nd

billion, 30 for 3rd billion, 15 for 4th billion, 12 for 5th billion and 9 for 6th billion…

 Population growth is slowing but still ~50 years the population should double…how?

Age Structure

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Human growth rates